Fly ash particles are a byproduct of coal fired power plants. These particles constitute about 10% of the total weight of the coal burned and, if not beneficially used, can be a substantial disposal problem.
The largest volume and highest value use of fly ash particles is as an admixture in Portland cement concrete. For use in concrete, the fly ash particles must meet stringent chemical and physical specifications, such as ASTM C-618. Often, the fly ash particles have a carbon content which precludes this beneficial use of the fly ash particles. As a result, these high carbon content fly ash particles must be disposed of at certain designated sites.
To utilize these fly ash particles, methods and apparatuses which use combustion processes to beneficiate fly ash particles by reducing the carbon content have been developed, such as those disclosed in U.S. Pat. Nos. 5,160,539 and 5,399,194, which are each herein incorporated by reference in their entirety. The beneficiated fly ash particles can be used to replace a portion of the cement in concrete, instead of requiring disposal in landfills.
Unfortunately, when the fly ash is beneficiated, gaseous compounds formed from sulfur in the fly ash are emitted into the environment in the exhaust gases. As environmental emission standards continue to be raised, new techniques for controlling sulfur emissions are required.
The basic chemical reactions that allow use of limestone (CaCO3) to capture gaseous sulfur compounds resulting from combustion processes are well known. For example, limestone mixed with water is sprayed into boiler exhaust gases to capture sulfur in many power plants in devices known as Flue Gas Desulfurization units (FGD) or more simply as wet scrubbers. However, these FGD units require the benefits to reaction kinetics of wet chemistry (i.e.—much of the reaction occurs in the liquid phase) to allow economic and efficient sulfur capture at gas inlet temperatures of about 300° F.
Another example is the use of limestone beds in fluid bed combustors to capture sulfur compounds that result from burning fuels, such as coal and petroleum coke. However, these combustors require the bed be primarily composed of limestone with the fuel being only a small fraction of total bed mass. More importantly, they require high temperatures, i.e. above about 1375° F., to optimize the sulfur capture reaction.
Graphs and tables of examples of sulfur capture v. temperature at 2.0, 2.5, and 3.5×stoichiometric ratio are shown in FIGS. 1A-1C. The data used to plot these graphs was obtained from, “Combustion: Fossil Power Systems,” published by Combustion Engineering, Inc., 3rd Edition, 1981, Page 24-23 which is herein incorporated by reference in its entirety. These graphs indicate that the optimum sulfur capture temperature for fluid bed combustors with limestone beds is at about 1550° F. The graphs also show that the optimum temperature is essentially unchanged by increasing the concentration of limestone. Limestone concentration is expressed as the molar ratio of calcium (Ca) to sulfur (S). This ratio represents the multiple of excess Ca used to ensure a high rate of sulfur capture. The graphs cover Ca/S molar ratios of 2.0, 2.5, and 3.5 which is a typical range for fluid bed combustors and show that overall sulfur capture rises with molar ratio, but that the optimum temperature remains essentially constant.
Further, the graphs show that sulfur capture efficiency falls off rapidly as temperature either increases or decreases from the optimum. This can be seen from tables in FIG. 1A-1C (data points taken from the Combustion Engineering reference noted above) which list temperatures and percentages of sulfur capture in fluid beds. The graphs in FIG. 1A-1C were derived using curve fitting techniques to extrapolate the temperature range of these data points. The extrapolation indicates that very little or no sulfur capture would be expected below about 1375° F. for a molar ratio of 2.0 as shown in FIG. 1A. Even at a molar ratio of 3.5, a similar analysis shows little or no sulfur capture would be expected below about 1300° F. as shown in FIG. 1C.